The formation and retention of charges of static electricity on the surface of most plastics is well known. Plastic materials have a significant tendency to accumulate static electrical charges due to low electrical conductivity. The presence of static electrical charges on sheets of thermoplastic film, for example, can cause the sheets to adhere to one another thus making their separation for further processing more difficult. Moreover, the presence of static electrical charges causes dust to adhere to items packaged in a plastic bag, for example, which may negate any sales appeal.
The increasing complexity and sensitivity of microelectronic devices makes the control of static discharge of particular concern to the electronic industry. Even a low voltage discharge can cause severe damage to sensitive devices. The need to control static charge buildup and dissipation often requires the total assembly environment to be constructed of partially conductive materials. It also may require electrostatic protective packages, tote boxes, casings, and covers be made from conductive polymeric materials to store, ship, protect, or support electrical devices and equipment.
The prevention of the buildup of static electrical charges which accumulate on plastics during manufacturing or use has been prevented by the use of various electrostatic dissipative (ESD) materials. These materials can be applied as a coating which may be sprayed or dip coated on the article after manufacture although this method usually results in a temporary solution. Alternatively these materials can be incorporated into the polymer during processing thereby providing a greater measure of permanence. However, the incorporation of these lower molecular weight electrostatic dissipative materials (antistatic agents) into the various polymers has its own limitations. For example, during the hot temperatures required during conventional processing many of the antistatic agents cannot withstand high temperatures and are damaged or destroyed, thereby being rendered useless with respect to their ESD properties. Also, many of the higher molecular weight ESD agents are not miscible with the base polymers employed, and if the refractive indices differ by more than about 0.02, there can be a substantial reduction in the transparency of the composition. These compositions may be unacceptable for transparent applications. For example, in an immiscible polymer blend where the dispersed phase particle size is greater than 0.1 micron the smaller the difference in the refractive indices between the additives and the base polymer the greater the clarity of the article made from the mixture.
A large number of antistatic agents are also either cationic or anionic. These tend to cause the degradation of plastics, particularly PVC, and result in discoloration or loss of physical properties. Other antistatic agents have significantly lower molecular weights than the base polymers themselves. Often these lower molecular weight antistatic agents possess undesirable lubricating properties and are difficult to incorporate into the polymer. Incorporation of the lower molecular weight antistatic agents into the polymers often will reduce the moldability of the base plastic because the antistatic agents can move to the surface of the plastic during processing and frequently deposit a coating on the surface of the molds, possibly destroying the surface finish on the articles of manufacture. In severe cases, the surface of the article of manufacture becomes quite oily and marbleized. Additionally, the lower molecular weight ESD agents often tend to lose their ESD capability due to evaporation, develop undesirable odors, and can promote stress cracking or crazing on the surface of an article in contact with the article of manufacture.
One of the known lower molecular weight antistatic agents is a homopolymer or copolymer oligomer of ethylene oxide. Generally, use of the lower molecular weight polymers of ethylene oxide or polyethers as antistatic agents are limited by the above-mentioned problems relative to lubricity, surface problems, or less effective ESD properties. Further, these low molecular weight polymers can be easily extracted or abraded from the base polymer thereby relinquishing any electrostatic dissipative properties.
There are several examples of high molecular weight electrostatic dissipative agents in the prior art. In general these additives have been high molecular weight polymers of ethylene oxide or a derivative thereof like propylene oxide, epichlorohydrin, glycidyl ethers and the like. It has been a requirement that these additives be high molecular weight materials to overcome the problems mentioned above. However, these prior art ESD additives result in articles having high haze values and thus are not transparent enough for some end uses.
Prior to the present invention the utilization of low molecular weight polyether oligomers as antistatic agents was impractical as these low molecular weight oligomers suffer from problems such as blooming.